The present invention relates to a pacemaker comprising a first electrode lead disposed in one atrium of a heart for stimulating atrial reactions. A second electrode lead is disposed in one chamber of the heart for stimulating and sensing chamber reactions. A stimulating pulse generator is provided for generating and emitting stimulating pulses to the atrium and the chamber via said first and said second electrode lead, respectively. A control device controls the emission of stimulating pulses and the sensing reactions. After each emitted atrial stimulating pulse, the control device starts an AV interval having a first duration, after the expiry of which a chamber stimulating pulse is emitted if no chamber reaction has been sensed during said AV interval.
A pacemaker of this type is disclosed in U.S. Pat. No. 4,312,355 and comprises a stimulating pulse generator which generates stimulating pulses, a first electrode which is disposed in one atrium of a heart, a second electrode which is disposed in one chamber of the heart, and a control device for controlling the different functions of the apparatus. Via the first electrode, stimulating pulses can be supplied to the atrium for stimulating an atrial reaction, and spontaneous atrial reactions can be sensed. In the same way, chamber reactions can be stimulated and sensed via the second electrode. The pacemaker according to the U.S. '355 patent has an inhibitory function, i.e. if a chamber reaction has been sensed within the AV interval after an atrial reaction (spontaneous or stimulated), no chamber stimulating pulse will be emitted. The same applies to atrial stimulations in relation to chamber reactions, i.e. if an atrial reaction has been sensed within the AV interval, no atrial stimulating pulse will be emitted.
Basically, a healthy heart functions as follows. The atrium contains the SA node or the sinus node. The sinus node is able itself to be depolarized at a regular rhythm, which may vary depending on factors such as physical activity and state of health. Depolarization of the sinus node results in a depolarization of the atria which then contract and pump blood into the chambers. Depolarization pulses continue via a plurality of conduction systems to the AV node, or the atrioventricular node, which is located in the area between atrium and chamber. The depolarization pulse is conducted via the AV node to the conduction systems in the chambers, which contract and pump the blood out into the systemic circulation and the pulmonary circulation, respectively. The atrium and the chamber are thereafter repolarized, and are then susceptible to the next depolarization pulse from the SA node.
In patients having a non-functioning, or a poorly functioning sinus node, a so-called sick sinus node syndrome, the atrium thus has to be stimulated regularly by the pacemaker. Even if the AV node has a normal function, a double-chamber pacemaker is sometimes implanted as a safety precaution, where the chamber electrode is used for stimulation only when temporary disorders or problems arise in the AV node, such as a delay of the AV conduction on account of vasovagal reactions.
Physiologically, the conduction time in the AV node is within the range 120-220 ms. In order not to stimulate the chamber unnecessarily, it is therefore necessary to program pacemakers with an inhibitory function for chamber stimulation using a long AV interval. AV intervals of up to 250 ms are not unusual.
A long AV interval entails that when a chamber stimulating pulse is actually emitted, the atria may already have been repolarized. There is then the risk that a retrograde conduction with ensuing depolarization of the atria follows the chamber stimulation. When the next atrial stimulating pulse is emitted, the atrium may be in its biological repolarization phase and then is unsusceptible to a normal stimulating pulse. Hence, there will be no depolarization or any conduction to the chamber. After the AV interval has expired, the chamber is therefore stimulated anew. The pacemaker may then be locked in a loop where the chamber is continuously stimulated after each AV interval, despite the fact that the patient would normally not need such stimulation. Apart from discomfort to the patient, there is an increase in the consumption of power from the battery in the pacemaker which then is discharged more rapidly than if the intended function of the pacemaker, i.e. to stimulate only the atrium, could be maintained.
In patients having a pacemaker with a frequency varying depending on the level of activity of the patient, the retrograde conduction time need not be very long before these problems occur.